PESIGN Cfl.tR.-tCTERISTlCS OF ELECTROMAGNETS 227 



>iittitiiiit tension is set up in the springs to insure a pressure of at least 

 15 jjranis between all contacts at the lime of closure. 



The ainoiuit of current and [lower required to operate each relay 

 is dependent Ufion the tension and nuniher of s()rinKS that must be 

 nio\ed antl the tlistancc through which this mo\ement takes place. 

 Relays or electromagnets operate most efficiently with the arma- 

 ture air-gaps set at the minimum required for the s;itif factory opening 

 and closing of the contacts. Consequently a method has been care- 

 full%- worked out for these relays in which the armature travel is 

 set in accordance with the requirements of the particular spring 

 condiination by the adjustment of the friction lock nut "X" shown 

 in Fig. 12. This setting of the armature insures a normal separa- 

 tion of contacts of approximately .010 inch and at least .005 inch 

 "follow" after closure of the contacts. The "follow" allows for a 

 certain amount of contact wear as well as insuring a slight wiping 

 action which gives a certainty of contact closure. The electrical 

 operating current requirements are figured and specified on the basis of 

 obtaining 20 grams pressure between all contacts; this margin being 

 allowed so that no undue hardship will be experienced in maintaining 

 the minimum requirement of 15 grams. 



The insulating materials used throughout have been carefully 

 studiwi and the best materials known to the present day art have 

 been used. Thus the wire used in the winding is insulated with a 

 high grade enamel and the insulating papers on the core are prac- 

 tically inert from an electrolytic corrosion standpoint. The coils 

 are covered with a serving of cotton, treated with unbleached shellac 

 which acts as a seal against moisture and protects the winding from 

 abrasion. The phenol fibre used on the spool heads and spring insu- 

 lators is much superior to hard rubber in regard to its ability to with- 

 stand a wide temperature range without appreciable expansion or 

 contraction. 



Ff)r this reason it is permissible to work these relays at higher 

 temperatures without danger of fire hazard or deterioration of the 

 insulation than relays insulated with hard rubber parts. These 

 higher temperature limits permit a wider usefulness of the relays in 

 circuits as well as economy in construction as the size of the coil 

 often depends on the necessary area for radiation and this area is 

 fixed by the permissible temperature range. 



Where the relays are to remain operated a considerable length of 

 time throughout the day the annual power charges become important 

 and the design of the winding and in some cases the size of the spool 

 must be altered to give the minimum annual charge. The group of 



